In a BGE RTS, how could I make a selection box to select Units?
Here's how to select people in general, by clicking on them with a button other then the one that sets the mouse tracker. Unfortunately, I don't know how to do a draw-box, but if someone else does, I would be super thrilled.

You need to add a mouse sensor, and set the field from the default Left-Mouse-Button to something else, like Middle-click, or Right-Mouse-Button. You also need a property on the cube; call it "active". This property needs to change to "True" when the cube is selected, and "False" when the cube is inactive; therefore, the Boolean variable type would be the best for this property, since there are only two different modes.
In order to change it, you need a "Property" actuator set to toggle (that way you can deselect also). In the field with the little bullet, (the property field) enter "Active" which is the name of the property you want changed. 
Connect the actuator to your Mouse sensor you created earlier.

Now when you click on a cube, it will set the property "Active" to true.
All you need to do is create another property sensor, set the evaluation type to "Equal" and enter true for the search, with Active as the property to be evaluated. connect this to the AND controller that allows your cube to steer toward the tracker and Viola!

Here's a blend to show you. hit Num5 to add people.